A wireless apparatus such as, for example, a base station or a user terminal in a wireless communication system includes an amplifier (power amplifier: hereinafter referred to sometimes as “PA”) for amplifying power of a transmission signal. In a wireless apparatus, a PA is caused to operate in the proximity of a saturation region of the PA in order to assure a high power efficiency of the PA. However, if the PA operates in the proximity of the saturation region, then the nonlinear distortion increases. Therefore, in order to suppress the nonlinear distortion of the PA to reduce adjacent channel leakage power (ACP), the wireless apparatus includes a distortion compensation unit for compensating for the nonlinear distortion of the PA.
As one of distortion compensation types used in the distortion compensation unit, a “pre-distortion (hereinafter referred to sometimes as “PD”) type” is available. The distortion compensation unit of the PD type improves the linearity of an output signal of the PA to suppress the distortion to occur in the output signal of the PA by multiplying a transmission baseband signal before inputting to the PA by a distortion compensation coefficient having an inverse characteristic to the nonlinear distortion by the PA in advance. By the multiplication of the transmission baseband signal by the distortion compensation coefficient, both of the distortion of an amplitude component of the transmission baseband signal and the distortion of a phase component of the transmission baseband signal are compensated for. In the following description, a signal after a transmission baseband signal is multiplied by a distortion compensation coefficient is sometimes referred to as “pre-distortion signal (PD signal).” Therefore, a PD signal is a signal distorted in advance in accordance with the inverse characteristic of nonlinear distortion by a PA before the PD signal is inputted to the PA. Further, in the following description, distortion of an amplitude component of a transmission baseband signal is sometimes referred to as “amplitude distortion,” and distortion of a phase component of a transmission baseband signal is sometimes referred to as “phase distortion.”
For example, some distortion compensation unit of the PD type includes a lookup table in which a plurality of distortion compensation coefficients are stored (such a lookup table is sometimes referred to as “distortion compensation table”). A distortion compensation unit that includes a distortion compensation table reads out a distortion compensation coefficient corresponding to an amplitude value of a transmission baseband signal inputted to the distortion compensation unit from the distortion compensation table and multiplies the transmission baseband signal by the distortion compensation coefficient. Each distortion compensation coefficient stored in the distortion compensation table is successively updated such that the error between the transmission baseband signal as a reference signal and a signal outputted from the PA and fed back to the distortion compensation unit (such a signal is hereinafter referred to sometimes as “feedback signal”) may be minimized.